Homologous radioimmunoassay and radioreceptor assay systems for mouse epidermal growth factor (EGF) will be employed to measure tissue concentrations of EGF in the developing mouse. Fetal and neonatal mouse skin will be utilized as a model experimental system for in vivo and in vitro studies. Major emphasis will be directed to the proposed regulation of EGF concentration and action in epidermis and dermis by developing endocrine systems; viz., the thyroid and glucocorticoid hormone axes. Hormonal modulation of EGF receptor number and affinity will be investigated in whole skin and epidermal membrane preparations and the results correlated with EGF levels determined by RIA in the same tissue homogenate supernatants. Hormone-EGF interactions including effects on cutaneous EGF biosynthesis will be examined using fetal and postnatal mouse skin in short term organ culture. This work will be complemented by characterization of thyroid nuclear receptor and glucocorticoid cytosolic receptor binding in newborn mouse epidermis and dermis.